1. Field of the Invention
The present invention relates to a projection exposure apparatus for use in the micropattern transfer in a lithographic process in the manufacture of large-scale integrated circuits or the like.
2. Related Background Art
For pattern transfer of the most-advanced large-scale integrated circuits there have widely been employed reduction projection exposure apparatus or steppers utilizing g-line spectrum (wavelength 436 nm) of ultra-high pressure mercury vapor lamps, but steppers utilizing an excimer laser employing XeCl, KrF, ArF etc. as the light source are being contemplated for achieving finer pattern transfer with a high throughput in the future. The projection lens system for imaging with the light of the excimer laser can be a wide-band lens which is color corrected for the light of a wide spectral range (for example about 0.4 nm) obtained by spontaneous oscillation or the excimer laser, or a narrow-band lens matching the light of excimer laser of a spectral range of 0.01 nm or smaller. The wide-band lens is composed of quartz and fluorite, and is free from speckles because there can be employed excimer laser light of a wide spectrum range and low coherency. Also the through-the-lens (TTL) alignment will be possible since it can be corrected in aberration for the wavelength not actinit to the photoresist. It is, however, difficult, with the present lens manufacturing technology, to obtain a lens having a transfer area of a diameter of 22 mm or larger and having a numerical aperture (N.A.) of 0.35 or larger, which is largely required for the mass production of large-scale integrated circuits.
On the other hand, the narrow-band lens is entirely composed of quartz, and can be satisfactorily produced with the most advanced manufacturing technology available presently. Consequently the lens of this type is expected for early commercialization.
In such narrow-band lens, there has not been available suitable technology for through-the-lens alignment, even if a resolution of 0.5.mu. or smaller is obtained in an area of a diameter of about 22 mm. This is firstly because the patterns on the wafer cannot be observed with a high resolution, as such narrow-band lens is anachromatized only for the spectrum of the excimer laser light for exposure so that the aberrations are not corrected for alignment with the non-actinic light. Secondly, even if the excimer laser light for exposure is used for illumination for the alignment, the wafer surface cannot be observed through the photoresist as such phtoresist generally has strong absorption.